Trees can live thousands of years but can't grow hundreds of meters. Tree biologists are discovering why

Transplanted to New York City, the tallest tree in the world would shade the Brooklyn Bridge. Moved to Pisa, it would be twice the height of the Leaning Tower. At 113 meters, this record California redwood begs a question: Why do some trees grow so tall? Scientists, of course, see the question from a different perspective: Why don't trees grow even taller? “This is one of the big mysteries in plant growth,” says Brian Enquist, a functional ecologist at the University of Arizona, Tucson.

Genetics clearly has something to do with tree height variations: You don't see many towering dogwoods, and conifers tend to top hardwoods. The environment also plays a key role; that redwood wouldn't be so giant in scrubland. But neither genetics nor environment can fully explain why, no matter the species, as a tree gets taller, its growth rate slows, sometimes dramatically. In Australia, mountain ash (Eucalyptus regnans) saplings can sprout more than 2 meters per year. By age 90, the tree is inching up just 50 centimeters per year, and by age 150, upward growth has ceased. And the gradual stalling of tree growth is not just an academic issue. Foresters care because maximum tree height is a good predictor of a stand's productivity, and environmentalists want to know the role of tree height and forest growth in the regulation of climate changes.

The obvious answer to why trees stop growing is that they simply get old and “feeble.” But new evidence seems to discount this cause, at least to some degree. Now, researchers—some of whom have been hoisted to canopies with construction cranes to take a look at what happens at the tops of trees—are focusing on water transport and photosynthesis. Newly published and unpublished results suggest that the function of water-conducting cells declines as a tree pushes ever higher. “Thanks to this work, the state of the field is changing rapidly,” says Karl Niklas, a plant biophysicist at Cornell University.

Size matters

Maurizio Mencuccini, a forest ecologist at the University of Edinburgh, U.K., has been retrieving the growing tips of old trees to test whether age-related genetic changes are at the root of maximum tree heights. He and his colleagues have just finished a study of ash, sycamore, Scots pine, and poplar trees to tease apart the effects of a tree's age and size on growth, as the two are intimately connected.

Mencuccini hypothesized that if age is the primary reason growth slows, an elderly tall tree's growth tips should still grow slowly when grafted onto young rootstock. Leaves and needles should look “old” as well. However, if tree size itself is the key to the changes seen in “aged” trees, then an old graft on young roots should resume growing fast and have the leaves of a much younger tree. As Mencuccini's group reported in the November Ecology Letters, growth tips from old trees resumed normal growth when grafted onto the rootstock. “Basically, it's size that matters, not absolute age,” says Mencuccini. But “we still don't fully grasp why size is so important in affecting tree physiology,” he adds.

Other tree researchers have been reevaluating a proposal dating back 50 years that looks to photosynthesis as the arbiter of tree height. At that time, the rationale was that extensive root or wood growth and respiration would eventually outpace the leaves' ability to produce enough energy to sustain those tissues. If that were the case, growth would become so slow and incremental that the tree couldn't keep up with natural losses such as die-back of the crown and would get stuck at a particular height. In the past decade, however, experiments have shown these energy-deficit explanations to be flawed. Neither roots nor woody growth hogged as much energy as researchers had thought.

In the 1990s, Michael Ryan, a forest ecologist now at the U.S. Department of Agriculture (USDA) Forest Service in Fort Collins, Colorado, and his colleagues found that leaves on smaller, younger trees are much more photosynthetically productive than leaves at the tops of taller trees. The reason, they surmised, might be that the higher leaves lack sufficient water, so he and Barbara Bond, a forest ecologist at Oregon State University, Corvallis, proposed what they called the hydraulic limitation hypothesis. “As a tree grows taller, it gets harder to pull water to the top,” and that shortfall curtails photosynthesis, summarizes Bond. Friction is the problem, she adds: The farther the water has to travel, the more resistance it encounters.

To check out their hypothesis, Ryan and Bond focused on stomata, tiny pores on the leaf's surface that can close to slow water loss that comes about as evaporation sucks water up the tree and into the air, leaving the leaves high and dry. But stomata also take in the carbon dioxide necessary for photosynthesis, and Ryan and Bond found that the stomata on the uppermost leaves close frequently, presumably because the top of the tree isn't getting sufficient water and needs to limit further loss through evaporation. This curtails needed carbon dioxide intake. As a result, “trees stop growing when their ability to transport water to their leaves becomes insufficient [for photosynthesis],” explains Roland Ennos, a biomechanicist at the University of Manchester, U.K.

Less water at the top of a tall tree also means lower hydrostatic pressure, or turgor, within cells, which is necessary for plant cells to expand. At some point, water pressure inside cells at the tops of trees may drop enough to stop cell growth directly. Bond, as well as Frederick Meinzer and David Woodruff, plant ecophysiologists at the USDA Forest Service in Corvallis, were among the first to show that this decrease in pressure might affect tree growth. And the role of hydrostatic pressure was borne out in 2004—at least in the world's tallest trees. George Koch, a tree biologist at Northern Arizona State University in Flagstaff, and his colleagues reported that redwoods need hundreds of kilograms of water a day to keep their cells thriving, and turgor in 110-meter-high needles was half that in 55-meter-high needles. Based on this trend, his team calculated that redwoods could not exceed 130 meters in height.

At 113 meters, this northern California redwood is the world's tallest known tree.

CREDIT: NATIONAL GEOGRAPHIC/GETTY IMAGES

Others are finding that connections between water-conducting cells may affect the ultimate height of trees. Unpublished data by Jean-Christophe Domec and his colleagues at Oregon State University, Corvallis, indicate that pores in these connections shrink to cope with the increased water tension. If the pores get too small, tree growth stalls. But, as Jarmila Pittermann and colleagues at the University in Utah, Salt Lake City, report on page 1924, the height at which resistance reaches this tipping point differs between conifers and hardwoods. The relatively larger pores in these connections in conifers allow for more water flow, potentially giving conifers a chance to tower over other types of trees.

But water-transport problems can't be the whole story. Ryan and his colleagues tracked photosynthesis, water flow, growth, and other parameters of eucalyptus seedlings in Hawaii for almost 7 years. The older, 25-meter trees grew much more slowly than the younger ones, Ryan and his colleagues reported last year. The work did demonstrate that photosynthesis slowed, but water was too plentiful to be the cause. “Our simple idea that getting water to the tree top limits height growth is not correct for all trees,” says Ryan.

Although it's clear from all these studies, says Ryan, “that taller trees are different physiologically from shorter, younger trees,” he and his colleagues still don't know how these differences stop tree growth. Solving that mystery remains a tall order.